Assuming that once started the car runs OK, then:-
If the starter turns the engine slowly, the battery is probably run down or you've got a duff earth. The easiest thing is to check the earth strap first for clean connections. If this is OK then I would first put the battery on charge or take the car for a long drive to recharge the battery. If as a result the starter turns the engine (when cold) reasonably well, you've got to ask how the battery lost charge in the first place. Could be knackered battery, duff alternator, duff driving characteristics (i.e. too much radio/lights whilst stationary).
Mark you, it could always be the starter motor but if this seems to engage without effort then it probably is OK.
I assume the car does run Ok once started ?